Polished Silver vs Sparkling Amethyst

How close are they?

ΔE 19.6, loose: different enough that you should pick on purpose. Sparkling Amethyst is a little darker, much more saturated than Polished Silver.

Can I use Sparkling Amethyst instead of Polished Silver?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Sparkling Amethyst is a little darker, much more saturated than Polished Silver.

Can I use Polished Silver instead of Sparkling Amethyst?

The same difference applies the other way round: Polished Silver is a little lighter, much more muted.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.